Below is a link to an interesting article about a possible cause of the decline of violence in the US over the last few years.
As we all know, correlation does not prove causation. However... I found the article to be interesting and so have provided the link. Enjoy.
http://www.motherjones.com/environment/2013/01/lead-crime-link-gasoline
In addition, here is a link to Steven Novella's blog discussing the above article. His view and the comments are useful.
http://theness.com/neurologicablog/index.php/lead-and-crime/
Dan